Thanks for raising this inquiry into your use case for displaying discussion titles. Unfortunately, I don't believe this is possible with the out of box widgets and tiles. They're intended to display only content the specific user has access to, so unless unauthenticated users (guests) are also able to view the referenced content, then it will not display for them using the out of box widgets and tiles. It may be possible to create a list of discussions manually, but this wouldn't be programmatic in nature and you'll need to maintain it by hand, so that's probably not the idea nor scalable way to do it.
if this is an important feature you'd like to address, you may want to look into potentially customizing this through a Jive Professional Services (PS) engagement. Let me know if you'd like to be put in touch with our PS team and I'll be happy to connect you.